xfs: factor out quotaoff intent AIL removal and memory free

AIL removal of the quotaoff start intent and free of both intents is
hardcoded to the ->iop_committed() handler of the end intent. Factor
out the start intent handling code so it can be used in a future
patch to properly handle quotaoff errors. Use xfs_trans_ail_remove()
instead of the _delete() variant to acquire the AIL lock and also
handle cases where an intent might not reside in the AIL at the
time of a failure.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>